Temporal and spatial redundancy can be exploited using predictions to make a compact representation of the video signal possible. Pixel prediction is an important part of video coding standards such as H.261, H.263, MPEG-4 and H.264 [1]. In H.264 there are three pixel prediction methods utilized namely intra, inter and bi-prediction. Intra prediction provides a spatial prediction of the current pixels block from previously decoded pixels of the current frame. Inter prediction gives a temporal prediction of the current pixel block using a corresponding but displaced pixel block in a previously decoded frame. Bi-directional prediction gives a weighted average of two inter predictions.
The inter prediction method of H.264 can achieve fractional-pel resolution in the motion estimation. A fixed half-pel filter with filter taps [1 −5 20 20 −5 1]/32 is first applicable to obtain initial half-pel resolution. A bilinear filter can then be applied on a full-pel sample and a half-pel sample to achieve quarter-pel resolution. These fractional-pel interpolation filters used in H.264 are fixed, implying the same filter taps are used regardless of which particular block that is encoded.
Many techniques have been suggested to improve the inter prediction over the years, for instance by employing adaptive interpolation filters. Such an approach has been suggested by first determining displacement vectors using the fixed H.264 interpolation filter for each pixel block to be coded. With the obtained displacement vectors, a calculation of adaptive filter coefficients of a two-dimensional non-separable adaptive Wiener interpolation filter [2], a separable adaptive interpolation filter [3], a directional adaptive interpolation filter [4] or an adaptive interpolation filter [5] is then done afterwards.
In H.264 inter-prediction, motion compensation accounts for a significant percentage of the compression efficiency. The motion information is derived at the encoder side and coded into the bit stream, where the motion information requires a major part of the bit stream. In this approach, the decoder can simply perform motion compensated prediction based on the decoded motion vector information. Usage of adaptive filters, though often improving the prediction accuracy, even further increases the overhead of the bit stream as the determined parameters of the adaptive filters need to be sent from the encoder to the decoder.